Progesterone has a proliferative effect in the normal breast and in breast cancers, and progesterone antagonists are growth inhibitory. Progestin effects are mediated by progesterone receptors (PR) and in breast cancer cells, there are two naturally occurring isoforms -- the 120 kDa B-receptors (PRB) and the N-terminally truncated 94 kDa A- receptors (PRA). since both homo- and heterodimers can form between them, three classes of receptor dimers (A:A, A:B, B:B) can bind to DNA, each having a potentially different transcriptional effect. To analyze the mechanisms of PR dimerization and the functional heterogeneity imparted by the three dimeric classes, in the context of progesterone agonist and antagonist actions in breast cancer, the following aims are proposed: Aim 1: To characterize progestin agonist action by analyzing PR dimerization, DNA binding, and transcription. We will use DNA binding domain (DBD) mutants of PR, and wild-type PRA and PRB, plus in vitro and in vivo functional assays, to characterize PR dimerization, to analyze the influence of DNA structure on dimerization, and to analyze the functional capacity of PR homo- and heterodimers when occupied by the agonist R5020. We will also construct mutants of a "leucine zipper" module likely to include the dimerization domain of PR, map its boundaries and key amino acids, and analyze its role in PR dimerization. Aim 2: To define the molecular mechanisms of two classes of progesterone antagonists. Three different antagonists inhibit transactivation by R5020, and one blocks binding of PR to DNA. We will analyze mechanisms of PRA and PRB homo- and heterodimerization, DNA binding and transactivation under the influence of the antagonists. We will analyze the role of progesterone response element (PRE) structure, promoter complexity, and cell specificity on transcriptional regulation by antiprogestins. We will address mechanisms involved in the dual agonist/antagonist actions of antiprogestins focusing on the possible role of cAMP in this functional switch. Aim 3: To analyze breast cancer cell proliferation as influenced by progestin agonists and antagonists, and to analyze a possible natural PR mutant. We will identify cell lines having phenotypically different responses to progestin agonists and antagonists, and use dual parameter flow cytometry (FCM) to quantitate proliferative effects and analyze heterogeneity of cell subpopulations as they are influenced by the progestins alone, or when combined with estradiol and tamoxifen. Hormone effects on cell cycle parameters will be measured, and improved FCM methods will be developed to quantitate S- phase, the non-proliferative G0 population, and a progestin marker. A PR-negative T47D line will be generated for use in transfection and as a control for the proliferation studies. Since mutant receptors may be involved in development of hormone resistance, we will characterize a putative natural PR mutant. We anticipate that these studies will begin to define molecular mechanisms of progestin actions in human breast cancers.